1. Technical Field
This application relates generally to an ignition device for internal combustion engines.
2. Background Art
Typical internal combustion engines, such as gasoline engines mounted in automobiles, are equipped with an ignition device with a spark plug to ignite fuel sprayed into a combustion chamber of the engine. For instance, a spark plug is known for use in such an ignition device which has a center electrode and a ground electrode and works to develop a plasma discharge between the center electrode and the ground electrode in response to application of a high-frequency voltage to the center electrode.
For example, Japanese Patent First Publication No. 2014-211148 teaches an ignition device equipped with an ignition circuit working to supply a high-frequency voltage to the spark plug. Specifically, the ignition circuit applies the high-frequency voltage between the center electrode and the ground electrode of the spark plug to create and develop a streamer discharge moving on a surface of an insulator porcelain between the center electrode and the ground electrode, so that an AC glow discharge or an arc discharge (each of which will also be referred to below as a glow discharge) is produced along a discharge path, as made by the streamer discharge, between the center electrode and the ground electrode.
The action of the streamer discharge as a precursor to the generation of the glow discharge requires application of very high voltage to the spark plug. Specifically, a voltage (i.e., a peak-to-peak voltage) of as high as 30 kVpp is required to develop the streamer discharge to produce the glow discharge. For this purpose, the ignition circuit is equipped with a step-up transformer which works to step-up a primary voltage, as developed by a high-frequency power supply, up to a secondary voltage and apply it to the spark plug. The ignition circuit is equipped with the high-frequency power supply which delivers a degree of the primary voltage (i.e., a high-frequency power) needed to develop the required voltage to the step-up transformer,
Once the glow discharge has been developed through the streamer discharge, the discharge will create an electrical conduction between the center electrode and the ground electrode of the spark plug, which results in a drop in impedance between the center electrode and the ground electrode, so that an electrical current actually flows therebetween. Accordingly, if the primary voltage (i.e., the high-frequency power) continues to be supplied, like when the streamer discharge is developed, to the step-up transformer to apply the secondary voltage to the spark plug, a high degree of current will flow between the center electrode and the ground electrode. In other words, an excessive current which is higher in level than that required to maintain the glow discharge will flow between the center electrode and the ground electrode, which results in a drop in durability of the spark plug arising from, for example, physical wear of the center and ground electrodes or deterioration of a resistor of the spark plug,
The above publication teaches dividing the current into a plurality of stages and delivering them in sequence to the spark plug in order to ensure the durability of the spark plug (i.e., the burst control). It is, however, difficult for such a method to maintain the glow discharge at a required level.